rockchip: gru: Add extra device-tree settings

Add some U-Boot-specific settings. These should really go in the
*u-boot.dtsi file, but it seems that rk3399 does not use that yet.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>